1. Field of the Invention
The preferred embodiments of the present invention are directed generally to downhole tools. More particularly, the preferred embodiments are directed to antennas that allow azimuthally sensitive electromagnetic wave resistivity measurements of formations surrounding a borehole, and for resistivity-based borehole imaging.
2. Background of the Invention
FIG. 1 exemplifies a related art induction-type logging tool. In particular, the tool 10 is within a borehole 13, either as a wireline device or as part of a bottomhole assembly in a measuring-while-drilling (MWD) process. Induction logging-while-drilling (LWD) tools of the related art typically comprise a transmitting antenna loop 12, which comprises a single loop extending around the circumference of the tool 10, and two or more receiving antennas 14A and 14B. The receiving antennas 14A, B are generally spaced apart from each other and from the transmitting antenna 12, and the receiving antennas comprise the same loop antenna structure as used for the transmitting antenna 12.
The loop antenna 12, and the receiving loop antennas 14A, B, used in the related art are not azimuthally sensitive. In other words, the electromagnetic wave propagating from the transmitting antenna 12 propagates in all directions simultaneously. Likewise, the receiving antennas 14A, B are not azimuthally sensitive. Thus, tools such as that shown in FIG. 1 are not suited for taking azimuthally sensitive readings, such as for borehole imaging. However, wave propagation tools such as that shown in FIG. 1, which operate using electromagnetic radiation or electromagnetic wave propagation (an exemplary path of the wave propagation shown in dashed lines) are capable of operation in a borehole utilizing oil-based (non-conductive) drilling fluid, a feat not achievable by conduction-type tools.
FIG. 2 shows a related art conduction-type logging tool. In particular, FIG. 2 shows a tool 20 disposed within a borehole 22. The tool 20 could be wireline device, or a part of a bottomhole assembly of a MWD process. The conduction-type tool 20 of FIG. 2 may comprise a toroidal transmitting or source winding 24, and two secondary toroidal windings 26 and 28 displaced therefrom. Unlike the induction tool of FIG. 1, the related art conduction tool exemplified in FIG. 2 operates by inducing a current flow into the fluid within the borehole 22 and through the surrounding formation 30. Thus, this tool is operational only in environments where the fluid within the borehole 22 is sufficiently conductive, such as saline water based drilling fluids. The source 24 and measurement toroids 26 and 28 are used in combination to determine an amount of current flowing on or off of the tool 20. The source toroid 24 induces a current flow axially within the tool 20, as indicated by dashed line 31. A portion of the axial current flows on (or off) the tool below toroid 28 (exemplified by dashed line 33), a portion flows on (or off) the tool body between the toroid 26 and 28 (exemplified by dashed line 35), and further some of the current flows on (or off) the tool at particular locations, such as button electrode 32 (exemplified by dashed line 37). Thus, the tool 20 of FIG. 2 determines the resistivity of a surrounding formation by calculating an amount of current flow induced in the formation as measured by a difference in current flow between toroid 28 and 26. As will be appreciated by one of ordinary skill in the art, the current measurement made by the toroids 26 and 28 is not azimuthally sensitive; however, for tools that include a button electrode 32, it is possible to measure current that flows onto or off the button 32, which is azimuthally sensitive.
Thus, wave propagation tools such as that shown in FIG. 1 may be used in oil-based drilling muds, but are not azimuthally sensitive. The conduction tools such as that shown in FIG. 2 are only operational in conductive environments (it is noted that the majority of wells drilled as of the writing of this application use a non-conductive drilling fluid), but may have the capability of making azimuthally sensitive resistivity measurements. While each of the wave propagation tool of FIG. 1 and conduction tool of FIG. 2 has its uses in particular circumstances, neither device is capable of performing azimuthally sensitive resistivity measurements in oil-based drilling fluids.
Thus, what is needed in the art is a system and related method to allow azimuthally sensitive measurements for borehole imaging or for formation resistivity measurements.